WASHINGTON >> The Federal Aviation Administration said Monday it is proposing a $12 million civil fine against Southwest Airlines for failing to comply in three separate cases with safety regulations related to repairs on Boeing 737 jetliners.

Beginning in 2006, Southwest made “extreme makeover” alterations to eliminate potential cracking of the aluminum skin on 44 jetliners, the FAA said. An FAA investigation determined that Southwest’s contractor, Aviation Technical Services Inc. of Everett, Washington, failed to follow proper procedures for replacing the fuselage as well as other work on the planes, the agency said. All of the work was done under the supervision of Southwest, which was responsible for seeing that it was done properly, the FAA said.

Southwest then returned the planes to service in 2009 and began flying them even after the FAA “put the airline on notice that these aircraft were not in compliance” with safety regulations, the agency said.

Southwest Airlines has 30 days to respond to the proposed fine. Usually FAA officials negotiate extensively with an airline in cases of large fines before settling upon an amount. However, regulators and airline officials sometimes are unable to reach an agreement and the airline contests the fine.

Southwest officials didn’t immediately reply to a request for comment.